1) Field of the Invention
The present invention relates to a thermal transfer image recording composite sheet, which will be referred to “image recording composite sheet” hereinafter, more particularly, the present invention relates to an image recording composite sheet in which an image recording sheet section is separably adhered to a release sheet section, and after images are thermal transfer-recorded on the image recording sheet section, the image recording sheet section can be separated from the release sheet section and adhered to a desired article, and which image recording composite sheet has a high resistance to roughening and denting of an image receiving layer due to a thermal shrinkage of a paper sheet substrate, on which the image receiving layer is formed, when the ink image recording sheet is heated imagewise by a thermal head of a thermal transfer printer, is capable of recording thereon ink images having a clarity and resolving degree compatible with those of silver salt photographic images, and is substantially free from denting under sheet-conveying nip roller pressure.
2) Description of the Related Art
A conventional thermal transfer image recording sheet is used to record images thereon by bringing a surface of an ink layer of an ink sheet into contact with a surface of an image receiving layer, comprising a dye-absorbing resin, of an image recording sheet, and thermally transferring portions of the ink or dye in the ink layer imagewise onto the image receiving layer surface by heating the ink layer imagewise with heat supplied from an imagewise heating means such as a thermal head.
For the recording, an ink ribbon having 3 coloring ink layers, namely a yellow-coloring ink layer, a magenta-coloring ink layer and a cyan-coloring ink layers or 4 coloring ink layers, namely a black-coloring ink layer in addition to the above-mentioned 3 color ink layers arranged repeatedly on a substrate film is used, and full-color ink images are formed by thermally transferring the yellow-, magenta-, cyan- and optionally black-coloring inks successively imagewise onto the ink image recording sheet and superposing the transferred coloring ink images one upon another.
The image recording sheet includes a thermal transfer image recording composite sheet comprising a release sheet section having a release layer formed on a sheet substrate for this section and an image recording sheet section having an adhesive layer formed on a surface of a sheet substrate for this section and separably adhered to the release layer of the release sheet section and an image receiving layer formed on an opposite surface of the sheet substrate for the image recording sheet section. In this type of the image recording composite sheet, after desired images are thermally recorded on the image receiving layer of the image recording sheet section, the image recorded sheet section can be released from the release layer of the release sheet section and adhered on a desired article. This thermal transfer recording composite sheet is usable as an adherable label.
The thermal transfer recording composite sheet must enable the ink images thermal transfer recorded thereon to have a high color density and a high uniformity and to be free from wrinkle marks transferred from the ink sheet or the ribbon. Also, the recording composite sheet needs to have a good moving property, through the printer.
Namely, the recording composite sheets must be smoothly supplied one by one into the printer, be superposed on the ink sheet, be separated from the ink sheet without fuse-adhering to the ink sheet, and be delivered from the printer without blocking the delivery of the recorded sheet from the printer. Further, after printing, the image-bearing recording sheet section can be easily and precisely released from the release sheet section. Furthermore, the released recording sheet section must have an appropriate compression modulus and hand.
Currently, with development of the thermal transfer printers, popularization of digital cameras, and improvements in the high degree of digital image treatment using computers, the quality of the resultant images is significantly enhanced, and thus the application of the ink image thermal transfer recording system in trade practice is expanded. For example, the ink image thermal transfer recording system is utilized in outputs of printing and proofreading of designs, outputs of images of endoscopies and CT scannings for medical treatments, and outputs of photographs of persons faces and calenders, for amusement, and of ID cards and credit cards for certification photographs. Also, with an improvement in the temperature-controlling technique for the thermal heads, an increase in recording speed of the ink image thermal transfer recording system is strongly required. For example, a printer capable of printing images on a A6 size sheet within a time of 30 seconds or less has appeared on the market. The requirement for an increase in recording speed will be further increased in the future.
Also, with the increase in the recording speed, new problems in gradation in color density of the recorded images, in precision and accuracy of the recorded images and in prevention of shearing in the ink images superposed one upon another occur. To obtain a good gradation in color density of the recorded images, the images in a wide range of the color density must be recorded with a recording energy in a narrow range and, to record images having a high color density with a low energy, the ink image recording sheet must have a high heat-insulating property. Also, to record ink images with high precision and accuracy, a close contact of the ink sheet with the thermal head and with the ink image recording sheet superposed on the ink sheet is necessary and the ink image recording sheet must have a good cushoning property.
In the full color printing procedure, to prevent the shearing in the recorded ink images superposed on each other, the ink image recording sheet is nipped, between spike-provided rolls and rubber rolls, to convey the sheet through the printer. In this case, to precisely convey the sheet at a high speed, the nipping of the sheet must be ensured by increasing the size of the spikes and/or the nipping pressure. This enhanced nipping causes a problem, that a plurality of dents and/or spike marks are formed on the image recording surface of the recording sheet, and thus the commercial value of the recorded sheet decreases, to occur.
In a thermal head type printer system, as a separate-adherable thermal transfer recording sheet capable of recording thereon ink images with a good quality, Japanese Unexamined Patent Publication No. 09-300,832 discloses an image recording sheet section having a foamed resin film layer on a surface of which an image receiving layer is formed, and a non-foamed resin film layer formed between an opposite surface of the foamed resin film layer and an adhesive layer. However, this type of the recording sheet has, as a whole, a high compressive modulus and is disadvantageous in that when the recording operation is carried out at a high recording speed while the recording sheet is conveyed by firmly nipping the recording sheet between spike-provided rolls and rubber rolls under an increased nipping pressure, to prevent shearing in printing, a plurality of undesirable dents and/or spike-marks are formed on the image recording layer surface of the recording sheet section.
In another type of known thermal transfer recording composite sheet, a recording sheet section is constituted from a substrate sheet (support) consisting of an oriented porous polymer film comprising, as a principal component, a polyester or polypropylene resin, an image receiving layer comprising, as a principal component, a dyeable resin and formed on a surface of the substrate sheet directly or through an intermediate layer, and an adhesive layer comprising an adhesive agent and formed on an opposite surface (free from the image receiving layer) of the substrate sheet.
The oriented porous polymer film is advantageous in that the thickness of the film is even, the softness of the film is appropriate, the heat conductivity of the film is lower than that of paper sheet comprising, as a principal component, a cellulose pulp fibers, and thus the recorded images on the image receiving layer have a high and uniform color density.
Generally, the color density of the recorded images can be increased by decreasing the apparent density of the film by, for example, increasing the number of pores and/or the size of the pores, to enhance the heat insulating property of the film. However, the decrease in the apparent density causes the mechanical strength and elastic modulus of the film to decrease and also, the resistance of the film to denting, due to the sheet-conveying rolls, to decrease.
When the image receiving layer is formed from a non-porous film comprising, as a principal polymer component, a polyester, the resultant surface layer exhibits a high heat resistance, a high tensile elastic modulus and enables the resultant recording sheet to exhibit a good hand as a label, and an increased resistance to the denting due to the sheet-conveying rolls. However, this polyester film layer causes a disadvantage in that the uniformity of contact of the film layer with the thermal head-through the ink sheet and the resistance to wrinkle formation on the recorded images are decreased.
Also, when the image receiving layer comprises, as a principal component, a polypropylene, is employed, the uniformity of contact of the film layer with the thermal head through the ink sheet is satisfactory. However, during the thermal transfer recording, the film layer shrinks so as to roughen the film layer surface and a plurality of dents are formed on the film layer due to the sheet conveying nip rolls.
Under the above-mentioned circumstances, as a substrate sheet for the image recording sheet section, an oriented porous polymer film comprising as a principal component, a polyester resin is widely employed.
However, the quality of the resultant conventional separate-adherable thermal transfer recording sheet is unsatisfactory, and must be improved.
Accordingly, there is a strong demand for a new type of thermal transfer image recording composite sheet having an image recording sheet section capable of uniformly contacting with the thermal head through an ink sheet, and thus of recording ink images with high precision and accuracy similar to those of silver-salt photographic images, and having high heat-insulating property, a low thermal shrinkage, and a high resistance to denting due to the sheet-conveying nip rolls.